ASCE 7-05 Unbalanced Snow Loads
ASCE 7-05 Unbalanced Snow Loads
(OP)
7.6.1 Says,
"Roofs with an eave to ridge distance, W, of 20' or less, having simply supported prismatic members spanning from ridge to eave shall be designed to resist an unbalanced uniform snow load on the leeward side equal to I x pg." (Importance Factor x Ground Snow Load)
Does this mean that regardless of slope, thermal and any other factors, the Unbalanced load will always control?
Importance factor will always be 1.
"W" will always be less than 20'
Slope will always be >2.38 and always < 70
Basically residential, gable roof homes.
I take the quote to mean that in this situation the roof flat out has to be designed to carry the Ground Snow Load.
Any thoughts?
"Roofs with an eave to ridge distance, W, of 20' or less, having simply supported prismatic members spanning from ridge to eave shall be designed to resist an unbalanced uniform snow load on the leeward side equal to I x pg." (Importance Factor x Ground Snow Load)
Does this mean that regardless of slope, thermal and any other factors, the Unbalanced load will always control?
Importance factor will always be 1.
"W" will always be less than 20'
Slope will always be >2.38 and always < 70
Basically residential, gable roof homes.
I take the quote to mean that in this situation the roof flat out has to be designed to carry the Ground Snow Load.
Any thoughts?






RE: ASCE 7-05 Unbalanced Snow Loads
The maximum moment for the unbalanced is wll/14.2.
So when the roof snow load is greater than 8/14.2 (0.563) the ground snow load, the balanced condition member resists a greater moment.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: ASCE 7-05 Unbalanced Snow Loads
Your signature is intriguing to me....what are you getting at?
I ask honestly.
Seems like an obvious thing, no?
RE: ASCE 7-05 Unbalanced Snow Loads
But there seems to be a disconnect in some EOR's opinions (as I see it, in some of the answers given in these forums) concerning the responsibilities between EOR's and Wood Truss Engineers. I get a bad feeling that they have never even look into what they are required to do for the wood trusses (or even other components) they are allowing to be used in their plans. So I decided to make this statement a signature.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: ASCE 7-05 Unbalanced Snow Loads
Mike McCann
MMC Engineering
RE: ASCE 7-05 Unbalanced Snow Loads
I would mention though that I have seen some truss packages with high girder truss loads to the exterior walls where either insufficient studs and footings were provided, or the window/door headers were overstressed. They got past the local jurisdiction with no structural review other than the local plans checker.
I would suggest that, in order to mitigate this particular case, it would be prudent by the truss designer to red flag these reactions with a prominent note - something to the effect of needing the attention of a licensed structural engineer prior to any permit being issued. This could be noted on the roof truss framing plan.
Mike McCann
MMC Engineering
RE: ASCE 7-05 Unbalanced Snow Loads
As for the red flags, unless you get it in as a code requirement it will never happen. Trusses are sold to the contractor or sub contractor, usually after the job has been permitted. Give them a set of truss designs that red flags the job, they will scream and then go to another truss company that does not red flag the job. We (engineers) should be complaining to the building departments and code authorities about getting better reviews and engineering for these low end building designs. Not trying to pass it off onto other engineers.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: ASCE 7-05 Unbalanced Snow Loads
RE: ASCE 7-05 Unbalanced Snow Loads
Guess I need to slow down and pay attention to the replies. You've all but answered my question. If there is an unbalanced condition then the I*Pg controls correct? The key word here being if. Aren't unbalanced loads determined by Exposure?
RE: ASCE 7-05 Unbalanced Snow Loads
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: ASCE 7-05 Unbalanced Snow Loads
RE: ASCE 7-05 Unbalanced Snow Loads
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: ASCE 7-05 Unbalanced Snow Loads
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: ASCE 7-05 Unbalanced Snow Loads
RE: ASCE 7-05 Unbalanced Snow Loads
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: ASCE 7-05 Unbalanced Snow Loads
I'm just trying to understand the wording. Should I determine the roof system of the cabin with the same loads as the house on the lake?
RE: ASCE 7-05 Unbalanced Snow Loads
I would always start with a conservative design and look at reducing it only if it becomes necessary. Keeping in mind that trees can be cut down during the life of the building.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.